More dependency cleanup (#825)

* Removes `rlp` dependency

* Enable warnings about unused crates in the CI

* Remove all the unused crates

* Make tests working again

substrate/core/primitives/src/lib.rs

@@ -62,8 +62,6 @@ extern crate core;
 extern crate wasmi;
 extern crate hash_db;
 extern crate hash256_std_hasher;
-#[cfg(feature = "std")]
-extern crate elastic_array;
 
 extern crate sr_std as rstd;